NORMAL FRATERNALLY ORIENTABLE GRAPHS SATISFY THE STRONG PERFECT GRAPHCONJECTURE

The chromatic number chi of a graph G is the minimum number of colors necessary to color the vertices of G such that no two adjacent vertice s are colored alike. The clique number omega of a graph G is the maxim um number of vertices in a complete subgraph of G. A graph G is said t o be perfect if chi(H) = omega(H) for every induced subgraph H of G. B erge's strong perfect-graph conjecture states that G is perfect iff G does not contain C2n+1 and CBAR2n+1, n greater-than-or-equal-to 2 as a n induced subgraph. In this paper we show that this conjecture is true for graphs which accept an orientation such that every complete subgr aph has an absorbing vertex and the set of predecessors (resp: success ors) of each vertex induces a complete subgraph. Also we obtain an equ ivalent version of the Strong Perfect Graph Conjecture.